So, I have a quite irritating/dangerous issue where I get a violent shaking of the steering wheel and pulsing through the brake pedal under heavy braking. The intensity of the shaking/pulsing varies with brake force applied, but the frequency of the oscillations are not affected while braking. The shaking/pulsing is experienced while braking at any speed (and at any brake force, with varied intensity) above 40mph, and shaking/pulsing mostly subsides under this speed (even under transition from any speed to below 40). The shaking in the steering wheel is rotational, with little/no lateral movement of the steering column. A somewhat loud clunking noise is heard from the steering column as the wheel shakes, which I think is from the u-joints of the column itself (but could be from elsewhere in the system).

I haven't necessarily ruled out warped rotors/uneven pads, but all 4 rotors and pads were just replaced with OEM in mid-November and this has been a noticeable issue starting less than a month after installation. The front subframe was also replaced at the same time with a brand new piece (no steering box mount reinforcement yet ), and front struts were replaced with Bilstein B4s in mid-September. I have inspected (albeit amateurishly) all front suspension components for play and everything is rock solid. The wheels have not been balanced for a bit over 2 years, and that's where I was looking to start first.

I should also note this: I have a faint shimmy of the wheel (identical to as described above) while cruising at highway speeds (between 75-80mph). This shimmy subsides almost entirely on either end of that range (not like I go over that speed anyway ).

Any advice/solutions are welcome! I feel incredibly confident in the handling of this car except for this "feature"

I’d be checking obvious things like whether the caliper bolts are loose. Also check the pads are wearing evenly.

Does it vibrate at speed or when cornering and not braking ?

I have not checked caliper bolts.. ever? Wow, shows how up to date I am. Will definitely inspect.
It vibrates violently while cornering if the brakes are applied at any speed (until below 40mph). The faint shimmy at speed does tend to subside somewhat while cornering, but is inconsistent. I've found that the second issue might actually be related to phasing of the front wheels: The shimmy in the wheel will "cycle" every 20 or so seconds while cruising at speed.

I do not think this is the case. I can still lock up both front wheels with relative ease, and do not have overheating brake issues while driving normally (only when getting after it on twisty roads, which is to be expected).

UPDATE: I have checked the caliper bolts and guide pins and there is no play in any bolts on either side. I didn't check for exact torque specs, but I figured any significant problems would be glaringly obvious at this point.

When you had the new subframe installed, were new upper and lower control arm bushings installed at that time? If not, I would take a look at them to see there condition. If there are any cracks in the bushings they need to be replaced. If they were replaced, when they installed them were they tightened down while the car was in the air or on the ground? The final tightening to the proper torque should be done under load with the wheels supporting the car.

Although unlikely given that you stated the rotors and pads were OEM, there could be an issue with the pads bedding properly with the rotors. Look at the rotors to see if the wear is even and smooth across the face and that there isn't any spotting on the rotors. Look at the back side of the rotors as well.

Also unlikely but possible, is that you have a rotor(s) that was a bad casting or that is not true and that as you have driven the car and the rotor(s) have heat cycled the problem has become more noticeable.

When you had the new subframe installed, were new upper and lower control arm bushings installed at that time? If not, I would take a look at them to see there condition. If there are any cracks in the bushings they need to be replaced. If they were replaced, when they installed them were they tightened down while the car was in the air or on the ground? The final tightening to the proper torque should be done under load with the wheels supporting the car.

AFAIK, none of the bushings/control arms were replaced when the new subframe was installed. I have inspected all the bushings and all seem to be in great shape. None of the protective rubber boots were torn, and all exposed bushings were free of cracks and had no signs of damage. I am also unsure of the method the shop used to re-install the subframe, but I assume (given the shop's reputation and knowing the owner) that installation was carried out as specified.

Although unlikely given that you stated the rotors and pads were OEM, there could be an issue with the pads bedding properly with the rotors. Look at the rotors to see if the wear is even and smooth across the face and that there isn't any spotting on the rotors. Look at the back side of the rotors as well.

Also unlikely but possible, is that you have a rotor(s) that was a bad casting or that is not true and that as you have driven the car and the rotor(s) have heat cycled the problem has become more noticeable.

If either is the case replacement is the only cure.

Each pad looks worn to the same degree (same thickness on all 4 pads). There were pad-shaped marks on the rotors, but it appeared to just be dust and was easily wiped away with my thumb, leaving no uneven surface on the rotors. This dust print was on both rotors on the outside and inside faces. But other than those, each face looked clean with no abnormalities. Maybe this would give me a good excuse to go down the Big Brake upgrade route..

I also found this: Upon visual inspection, the pads had not come off the rotor entirely. Earlier this morning, it was the inside pad on both sides (on my 1-piston calipers, the piston is on the inboard side of the caliper), but this afternoon (right now), both pads on both sides are gripping the rotor slightly. I assume this is because the power-assist system is not activated and they can't return to 100% off the pad, and I don't notice excessive heat buildup while driving, so this seems like a non-issue. But this has been the case for as long as we've owned the car (approaching 3 years) and none of the 4 shops it has seen (and with some frequency) have made any mention of this.

You may very well be right. I'm surprised it didn't happen before the new subframe went in, but I guess we just wanted an operational daily driver again rather than waiting longer. (after the car had been in the shop for 3 weeks )

if the pads are not coming off the disc fully for whatever reason (and one common cause is a faulty MC BTW), you will quickly get a runout-induced disc thickness variation (DTV) because the pads will kiss (and wear) the high spots on the disc.

A DTV of 0.0005" (yes, half a thousandth of an inch) is enough to cause the symptoms you are seeing. There may well be other faults too, but that'll do it.

The reason the shuddering is so bad at speed is because the suspension components can resonate. It doesn't take very much force variation to drive the resonance.

Disc warping or uneven thickness. I have had this on two of my cars in the past (911 Turbo and Monaro VXR500) and it can only be resolved by either new discs or having the discs skinmmed on the car: https://skimmydiscs.co.uk

if the pads are not coming off the disc fully for whatever reason (and one common cause is a faulty MC BTW), you will quickly get a runout-induced disc thickness variation (DTV) because the pads will kiss (and wear) the high spots on the disc.

A DTV of 0.0005" (yes, half a thousandth of an inch) is enough to cause the symptoms you are seeing. There may well be other faults too, but that'll do it.

The reason the shuddering is so bad at speed is because the suspension components can resonate. It doesn't take very much force variation to drive the resonance.

cheers

Wow, I definitely wouldn't have been able to guess the MC on my own. Thank you! That figure does seem extraordinarily small, but I guess it's not so small considering how relatively thin each disk surface is. And totally hear you on suspension part resonance. Audio and acoustics have taught me much about resonance frequencies and how finicky they can be

Disc warping or uneven thickness. I have had this on two of my cars in the past (911 Turbo and Monaro VXR500) and it can only be resolved by either new discs or having the discs skinmmed on the car: https://skimmydiscs.co.uk

I was hoping they wouldn't be, but I guess the truth is the truth . I'll see about having them resurfaced and having the MC replaced (with an E32 750i if I can find one around here). Thankfully resurfacing should work a treat since they're hardly 3000 miles new.

Thanks all for the help! I'll report back with results/info as they come

Disc warping or uneven thickness. I have had this on two of my cars in the past (911 Turbo and Monaro VXR500) and it can only be resolved by either new discs or having the discs skinmmed on the car: https://skimmydiscs.co.uk

+1, Definitely sounds like a warped rotor(s)

81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.

you can check very easily for run-out on the discs, but the tolerances for warping are huge by comparison with DTV... you need two things to get DTV

1) brake pads that don't clear the discs properly
2) any small amount of run-out (there is no lower limit if the pads are touching)

My suggestion is that you first fix the fault that means that the disc pads don't clear the disc properly, and check the discs for run out. If you have a mild amount of DTV, the rotors will (with any moderately abrasive pad) soon clean up in normal use. However if the DTV is bad enough, the discs will need to be replaced or remachined.

I had the same exact problem. Mine went away after replacing the upper control arms. My control arms were original and 300k miles old and had absolutely no bushing left on it hence the bad vibrations. I always heard people call it a shimmy but it can be magnitudes worse if left untreated like in my case.

My problem has actually somewhat subsided and my symptoms have narrowed down. My bushings are all still good, but now I have a slight grinding/chirping sound coming from the front brakes like they're not fully disengaging (indicitive of a dying master cylinder). I haven't been driving as much over the summer, to the priority of replacing it has gone down since the wheel doesn't vibrate hardly at all anymore. But when I so replace it, an E32 735i MC will be the replacement.

Keep it simple. Look for runout on the rotor as Brucey said and one other thing check your tire air pressure. I once had this on a car so bad it would almost tear the steering wheel out of your hand and it was air pressure in a tire. If not that start looking for your worst night mare going from simplest to more complex as you work your way through

81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.

if the MC is going bad, the usual thing I have seen (to start with, anyway) is that the behaviour is different when the engine is cold vs when the engine is hot.

Typically what happens is that shaking under braking (unless it has already made the discs very bad) is a lot less severe when the engine is cold, and when the engine is hot the problem is worse.

The reason this happens is that the usual MC problem is temperature sensitive; basically the MC doesn't return fully but typically only when the MC itself is hot. Normally there is a change in the pedal feel; when the MC is sticking there is often a dead spot at the top of the pedal travel and once there is some pedal resistance, it feels different to normal.

On my car the MC changed slowly; took two or three years to turn from a problem that only manifested itself once or twice a year (when moving slowly in traffic) to something that afflicted the majority of journeys whenever the weather was warm.

If the MC is going bad in this way, changing the UCA bushes is arguably treating the symptom, not the cause. It may well make a difference (and it is unlikely to make things worse) and old bushings may need changing anyway, but it won't fix an underlying problem if there is one.

Just had mine fixed. The issue was rather simple - a totally perished ball joint on the passenger control arm. My mechanic did not spot it at first because when everything is mounted it seemed pretty tight. But once the control arm was removed, the ball joint practically fell apart. I suspect it was original part from 1986. Pretty cheap fix and now car handles and stops as it should.

Hi OnTheFritz,
just a thought, your photo at bottom of your posts shows an e24 with, i think, e38 wheels. If this is the case the hubs on the e24 are smaller than the centre holes in the e38 wheels and require spacer shims. These can be plastic or preferably steel. If the shims/spacers are not fitted you can get severe steering shake when braking.

You may of course already know this, and have the shims fitted already, but sometimes odd things like this get missed.

No need to worry, these are just the regular 15in OE wheels for the E24. I’ve double and triple checked my pig bolts being tight, tire pressures, etc. I get more symptoms through the brakes (e.g. a pulsing squeak while driving, speeds up/slows down with speed, changes while cornering/going over bumps) than steering anymore

The last time they would've been lubed would be when the disks and pads were replaced last October, but I don't remember if they were lubed or not. They at least weren't wiggling around when I checked them last, but that might have changed since. What kind of grease is best to use there?